34 Flow Level Pressure

Ultrasonic Flow Meter Proves its Worth on Vacuum Tower Bottoms

Operating at temperaturesof around 350°C, the bottomof a vacuum distillation toweris no place for delicateinstrumentation. To measurethe flowrate of vacuumresidue or bitumen, refinerstherefore traditionally rely ondifferential-pressure (DP)devices such as orifice platesor venturis. These are simple,

robust, and economical in line sizes above 200 mm.

Unfortunately, simplicity and robustness do notalways imply accuracy or reliability. The biggestproblem with all DP devices is the need for a pair ofimpulse lines connecting the sensor to the DPtransmitter. Impulse lines often become plugged withparticles of solid material such as coke, or bubbles ofgas. In bitumen service, blockages are mostcommonly due to frozen product – a particularproblem in cold climates, despite trace heating.

Faced with unreliable orifice plates, one refinersought a better way to measure the flow of bitumenleaving a vacuum tower on its way to a delayed coker.In this case the product starts life as crude bitumenwhich is diluted with lighter hydrocarbons to create asynthetic crude. The lessons learned, however, applyequally well to refined bitumen produced in thevacuum distillation of conventional crude.

Versatile TechnologyThe refining company decided to try a clamp-onultrasonic flow meter as a way to make flowmeasurement more reliable. The main rationale for theexperiment was that because ultrasonic flow metersare non-invasive and do not require impulse lines, theycannot become clogged or blocked. Other importantfeatures of this flow measurement principle are that itis independent of system pressure, works withpractically any fluid, and remains accurate over awide turndown range (see boxes).

The company first carried out several tests with aportable ultrasonic flow meter, allowing it to select thebest transducer type and positions for the job in hand.Once the engineers were confident that the newtechnology would perform well, they installed apermanent clamp-on ultrasonic flow meter on the 400mm pipework connected to the bottom of thevacuum tower.

The complete system comprised two sets of FLEXIMType G low-frequency ultrasonic transducers with high-temperature mountings, and a Fluxus® ADM 7407transmitter with two input channels . The transmittercommunicates with the refinery’s distributed controlsystem (DCS), making it easy to compare the readingsfrom the ultrasonic flow meter with those of the flowtransmitters linked to the three original orifice plates(Figure 1).

Figure 1: The FLEXIM ultrasonic flow meter (A) in position onthe 400 mm discharge line from the bottom of the vacuumtower. B, C and D are the original orifice plates with flowtransmitters

Sound PerformanceThe trial took place from December 2008 to April2009.

Figure 2, which represents a period of 24 hours, showsthat the FLEXIM flow meter (A, red line) providedreliable and consistent readings, albeit with short-term spikes caused by the presence of gas bubbles.Orifice plate B (green), in contrast, was noisy andunreliable due to problems with freezing of theimpulse lines. Ignoring noise, the readings from theultrasonic flow meter agreed well with those fromorifice plate B.

The flowrate as calculated by subtracting thereading of orifice plate D (light blue) from that oforifice plate C (dark blue) (Figure 1) was consistentlyhigher by around 10% (44 bbl/h) than that measuredby the FLEXIM flowmeter and orifice plate B. Theaccuracy of the FLEXIM flow meter under idealconditions is ±1.6% of reading; the trial did not attemptto establish its accuracy under real plant conditions,but this is likely to be considerably better than that ofan orifice plate and DP transmitter.

Figure 2: The FLEXIM flow meter (red line) agrees well withorifice plate B (green), but is much more reliable. Flowratetrends shown by orifice plates C (dark blue) minus D (lightblue) show a consistent discrepancy of around 10%

Figure 3 shows the same comparison over the firstmonth of the trial. The reading from the ultrasonic flowmeter shows spikes, occasionally very large, due togas bubbles – but it is clearly more reliable and stablethan the measurements from orifice plates B and C.

Figure 3: Over one month of challenging conditions, thereading from the FLEXIM flow meter (red) is much more stablethan that of orifice plates B and C (green and blue,respectively)

Figure 4 shows the results for another one-monthperiod, this time between 15 March and 15 April. Bythe beginning of April ambient temperatures areabove freezing, and the stability of the readings fromorifice plates B and C improves accordingly.Regardless of temperature, however, the FLEXIM flowmeter is more stable than either of the two main orificeplates.

Figure 4: As ambient temperatures climb above freezing, theorifice plates become more reliable, though still not as reliableas the FLEXIM flow meter

Permanent PlacementBy the end of the trial the refiner was sufficientlyimpressed with the ultrasonic flow meter to make it apermanent installation, further increasing measurementreliability and reducing maintenance costs.

The company concluded that for bitumen and high-temperature service the FLEXIM clamp-on ultrasonicflow meter is more reliable than an orifice plate with aDP transmitter. This is true at all times, but especiallyduring freezing weather, when impulse lines areespecially likely to block.

The fact that it can be installed without shutting downprocess makes the ultrasonic flow meter ideal forretrofitting to existing plants.

Process conditions at the bottom of vacuum distillation towers make flow measurement challenging. In this case study, a clamp-onultrasonic flow meter proves more reliable than the traditional orifice plate.

Rainer Wetzel, Sales Director, FLEXIM GmbH Wolfener Str. 36, 12681 Berlin, Tel: 0049-172 3299004 Email: rwetzel@flexim.com

August/September 2010

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35Flow Level Pressure

The Author: Rainer Wetzel is Sales Director at FLEXIM GmbH andhas over 20 years of process instrumentationexperience, especially in the field of flowmeasurement. Since several years Mr. Wetzelanalyses oil & gas processes for possible FLEXIMoperations.

How ultrasonic flow meters workUltrasonic flow meters use beams of high-frequencysound to measure the velocity of moving fluids. Theycome in two kinds: Doppler and transit-time.

Doppler flow meters measure the frequency shift thatoccurs when ultrasound is reflected by bubbles orparticles in the moving fluid – the same principle as aradar speed gun.

Transit-time (time-of-flight) flowmeters like the FLEXIMdesign rely on the fact that the apparent velocity ofultrasound waves through the fluid depends on whetherthey are travelling with or against the direction of flow.The ultrasound transmitter and receiver can bemounted on opposite sides of the pipe, or on the sameside; in the second case they are arranged so that theultrasound reflects off the far wall of the pipe. Alternateultrasound pulses travel in opposite directions: once withthe flow and once against it.

Since the velocity of sound in the process fluid is known,the difference in transit time (ΔT) in the two flowdirections gives the average flow velocity across thepipe. When the pipe diameter is known, it also gives thevolumetric flowrate.

August/September 2010

Ultrasound Advantages

The ability of ultrasound to travel through pipe walls and other solid materials allows the ultrasonic transducers to be attached to the outside of the pipe.

This brings some significant advantages:

• The flow meter can be installed without shutting down the process.

• Installation is quick and simple, with no need for drilling, cutting or welding, no reduction in the integrity of the pipework, and no need for impulse lines or trace heating.

• The inside surface of the pipe remains smooth, with nothing to cause blockages or increase pressure drop.

• With no direct contact between the transducers and the process fluid, erosion and corrosion are avoided, and maintenance needs reduced.

• Corrosive fluids, high temperatures and high pressures can all be handled safely. Special mounting brackets allow FLEXIM transducers to work reliably at process temperaturesup to 400°C.

• With no moving parts, the transducers are highly reliable.

• Multi-beam technology, with two input channels and two sets of transducers, increases accuracy by averaging flow velocity in two directions across the pipe.

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